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Autori: I. L. Prejbeanu
In this work, two different aspects of the magnetic properties in systems of small dimensions have been investigated.
The first aspect concerns the domain wall magnetoresistance in monocristallin cobalt nanowires of rectangular section. These nanowires were fabricated by electron-beam lithography and dry etching from epitaxial cobalt thin films with strong in-plane uniaxial anisotropy. The reduction of the lateral size of the system influences drastically the distribution of the magnetization. Different micromagnetic configurations are obtained in wires, depending on their orientation, parallel or perpendicular, with respect to the easy magnetocristalline axis. These configurations are strongly affected by the magnetic history. The magnetization reversal in the wires was studied by magnetotransport measurements and interpreted in the context of models of domain walls scattering and conventional galvanomagnetic effects in ferromagnetic materials. The domain wall magnetoresistance obtained from this study is positive and increases at low temperature.
The second part is dedicated to the study of arrays of polycrystalline circular cobalt dots, fabricated by nanoimprint. For the case of widely spaced arrays, where the magnetostatic interactions between dots are negligible, different magnetization reversal mechanisms were identified, as a function of the dot dimensions: a coherent rotation of the magnetization and the formation of one or two vortices. For a dense array, the magnetostatic interactions generate collective magnetization reversal phenomena. These interactions lead to the formation, by an avalanche mechanism, of single-domain state chains or vortex-state chains, with identical rotation sense. Furthermore, they give rise to a quadratic anisotropy of the reversal process, and namely of the vortex nucleation field.
Cuvinte cheie: epitaxial thin films, magnetic nanostructures, domain and domain walls, magnetic force microscopy, galvanomagnetic effects, domain wall magnetoresistance, magnetostatic interactions, micromagnetics